Diffuse reflector

ABSTRACT

The present invention relates to a diffuse reflector comprising a nonwoven sheet of fibers wherein in the nonwoven sheet has a reflectance and a delamination selected from the group consisting of a reflectance of at least about 97.0% and a delamination of at least about 0.18 N/cm, a reflectance of at least about 96.0% and a delamination of at least about 0.49 N/cm, and a reflectance of at least about 95.0% and a delamination of at least about 0.70 N/cm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a diffuse reflector comprising anonwoven sheet for use in products such as televisions and lightingrequiring high reflectance while resisting sheet delamination.

2. Description of the Related Art

Nonwoven sheets with high reflectance can be used in light reflectiveend uses. Nonwoven sheets must be bonded for structural integrity and toprevent delamination. However, the bonding process lowers thereflectance. Coatings can be applied to the bonded nonwoven to increasethe reflectance.

It would be desirable to have a diffuse reflector made from a nonwovensheet having high reflectance while resisting sheet delamination andwithout the need for a coating.

SUMMARY OF THE INVENTION

The present invention relates to a diffuse reflector comprising anonwoven sheet of fibers wherein in the nonwoven sheet has a basisweight of from about 30 grams per square meter (gsm) to about 275 gsmand has a reflectance and a delamination selected from the groupconsisting of a reflectance of at least about 97.0% and a delaminationof at least about 0.18 N/cm, a reflectance of at least about 96.0% and adelamination of at least about 0.49 N/cm, and a reflectance of at leastabout 95.0% and a delamination of at least about 0.70 N/cm.

DETAILED DESCRIPTION OF THE INVENTION Definition of Terms

The term “nonwoven” or “web” as used herein means a structure ofindividual fibers or threads that are positioned in a random manner toform a planar material without an identifiable pattern, as in a knittedfabric.

The term “plexifilamentary fibers” as used herein means athree-dimensional integral network or web of a multitude of thin,ribbon-like, film-fibril elements of random length and with a mean filmthickness of less than about 4 micrometers and a median fibril width ofless than about 25 micrometers. The average film-fibril cross sectionalarea if mathematically converted to a circular area would yield aneffective diameter between about 1 micrometer and 25 micrometers. Inplexifilamentary structures, the film-fibril elements intermittentlyunite and separate at irregular intervals in various places throughoutthe length, width and thickness of the structure to form a continuousthree-dimensional network.

The term “polymer” as used herein, generally includes but is not limitedto, homopolymers, copolymers (such as for example, block, graft, randomand alternating copolymers), terpolymers, etc., and blends andmodifications thereof. Furthermore, unless otherwise specificallylimited, the term “polymer” shall include all possible geometricalconfigurations of the material. These configurations include, but arenot limited to isotactic, syndiotactic, and random symmetries.

The term “polyolefin” as used herein, is intended to mean any of aseries of largely saturated polymeric hydrocarbons composed only ofcarbon and hydrogen. Typical polyolefins include, but are not limitedto, polyethylene, polypropylene, polymethylpentene, and variouscombinations of the monomers ethylene, propylene, and methylpentene.

The term “polyethylene” as used herein is intended to encompass not onlyhomopolymers of ethylene, but also copolymers wherein at least 85% ofthe recurring units are ethylene units such as copolymers of ethyleneand alpha-olefins. Preferred polyethylenes include low-densitypolyethylene, linear low-density polyethylene, and linear high-densitypolyethylene. A preferred linear high-density polyethylene has an upperlimit melting range of about 130° C. to 140° C., a density in the rangeof about 0.941 to 0.980 gram per cubic centimeter, and a melt index (asdefined by ASTM D-1238-57T Condition E) of between 0.1 and 100, andpreferably less than 4.

Description

The present invention is directed to a diffuse reflector comprising anonwoven sheet of fibers wherein in the nonwoven sheet has a combinationof high reflectance and good delamination resistance.

The diffuse reflector of the present invention can be made by flashspinning a plexifilamentary fiber and the resulting nonwoven sheetgenerally by using the disclosures in U.S. Pat. No. 3,081,519 to Bladeset al., U.S. Pat. No. 3,227,794 to Anderson et al. and U.S. Pat. No.3,860,369 to Brethauer et al., which are hereby incorporated byreference, with the process exception as noted herein. The flashspinning process produces a flash spun nonwoven sheet ofplexifilamentary fibers. However, it has been found that flash spinningat low spinning temperatures produces plexifilamentary fibers withunexpectedly high surface area and the resulting bonded nonwoven sheethas high reflectance with good delamination resistance.

A method of forming a flash spun plexifilamentary fiber comprisingproviding a solution of 12% to 24%, by weight of the solution, ofpolyethylene and a spin agent consisting of a mixture of 68 weightpercent normal pentane and 32 weight percent cyclopentane; or 75 weightpercent normal pentane and 25 weight percent cyclopentane and flashspinning the solution at a spinning temperature from about 165° C. toabout 175° C. to form the flash spun plexifilamentary fiber.

The flash spun plexifilamentary fiber can be made with any suitablecombination of polymer and spin agent for flash spinning. A usefulpolymer is a polyolefin polymer, preferably polyethylene.

The nonwoven sheet was made by collecting the flash spunplexifilamentary fibers to form a sheet followed by bonding.

The nonwoven sheet has a reflectance and a delamination selected fromthe group consisting of: a reflectance of at least about 97.0% and adelamination of at least about 0.18 N/cm, a reflectance of at leastabout 96% and a delamination of at least about 0.49 N/cm, and areflectance of at least about 95.0% and a delamination of at least about0.70 N/cm. It is preferred not to have an optical brightener, however acoating with an optical brightener may be added to the nonwoven sheet toimprove the reflectance. For example, titanium dioxide with a binder canbe coated onto the nonwoven sheet.

The nonwoven sheet of the present invention has a preferred basis weightof from about 30 gsm to about 275 gsm. An increase in basis weight cangive rise to an increase in reflectance.

TEST METHODS

In the non-limiting Example that follows, the following test methodswere employed to determine various reported characteristics andproperties. ASTM refers to the American Society of Testing Materials.

Reflectance was obtained from single nonwoven sheets. A Model SP64 meteravailable from X-Rite, Incorporated, 4300 44^(th) St SE, Grand Rapids,Mich., 49512, USA is placed on the sample and the reading is obtained.The output is a percent reflectance at a wavelength of 550 nm.

Delamination Strength of a sheet sample is measured using a constantrate of extension tensile testing machine such as an Instron table modeltester. A 2.54 cm by 20.32 cm sample is cut such that its long directionis parallel to the machine direction and is delaminated approximately3.18 cm by inserting a pick into the cross-section of the sample toinitiate a separation and delamination by hand. The delaminated samplefaces are mounted in the clamps of the tester which are set 2.54 cmapart. The tester is started and run at a cross-head speed of 5.08cm/min. The computer starts picking up force readings after the slack isremoved in about 1.3 cm of crosshead travel. The sample is delaminatedfor about 14 cm during which force readings are taken and averaged. Theaverage delamination strength is the average force divided by the samplewidth and is expressed in units of N/cm. The test generally follows themethod of ASTM D 2724.

Basis Weight was determined according to ASTM D-3776 and reported ingsm.

EXAMPLE

Hereinafter the present invention will be described in more detail inthe following example.

Example 1

Example 1 represents a process for making a plexifilamentary fiber andresulting nonwoven sheet of the present invention. Plexifilamentaryfibers and resulting nonwoven sheet were made from flash spinningtechnology as generally disclosed in U.S. Pat. No. 3,081,519 to Bladeset al., U.S. Pat. No. 3,227,794 to Anderson et al. and U.S. Pat. No.3,860,369 to Brethauer et al. Plexifilamentary fibers were flash spunfrom a 19 weight percent concentration of high density polyethylenehaving a melt index of 0.7 g/10 min (measured according to ASTM D-1238at 190° C. and 2.16 kg load) and 3.5% TiO2 in a spin agent of 68 weightpercent normal pentane and 32 weight percent cyclopentane at a solutiontemperature of 175° C. A nonwoven sheet was made by collecting theplexifilamentary fibers into a sheet and whole surface bonding the sheetbetween two pre-heated rolls at 132.2° C., two pairs of bond rolls at133.3° C., one roll for each side of the sheet, with backup rolls madeby formulated rubber that meets Shore A durometer of 85-90, (with 34.5bar hydraulic pressure) and two chill rolls to form the bonded nonwovensheet. The nonwoven sheet (basis weight, 90.2 gsm) had a reflectance of97.2% and a delamination of 0.49 N/cm.

Example 2

Example 2 was made the same as Example 1, except the nonwoven sheet wasmade by collecting the plexifilamentary fibers into a sheet and wholesurface bonding the sheet between two pre-heated rolls at 126.7° C. and121.1 C, two pairs of bond rolls at 133.3° C. and 137.8 C, one roll foreach side of the sheet, with backup rolls made by formulated rubber thatmeets Shore A durometer of 85-90, (with 34.5 bar hydraulic pressure) andtwo chill rolls to form the bonded nonwoven sheet. The nonwoven sheet(basis weight, 89.2 gsm) had a reflectance of 97.1% and a delaminationof 0.30 N/cm.

Example 3

Example 3 was made the same as Example 1, except the plexifilamentaryfibers were flash spun from a 19 weight percent concentration of highdensity polyethylene having a melt index of 0.7 g/10 min (measuredaccording to ASTM D-1238 at 190° C. and 2.16 kg load) and 5.0% TiO2 in aspin agent of 75 weight percent normal pentane and 25 weight percentcyclopentane at a solution temperature of 175° C. A nonwoven sheet wasmade by collecting the plexifilamentary fibers into a sheet and wholesurface bonding the sheet between two pre-heated rolls at 126.7° C. and132.2 C, two pairs of bond rolls at 142.2° C. and 137.8 C, one roll foreach side of the sheet, with backup rolls made by formulated rubber thatmeets Shore A durometer of 85-90, (with 34.5 bar hydraulic pressure) andtwo chill rolls to form the bonded nonwoven sheet. The nonwoven sheet(basis weight, 119.4 gsm) had a reflectance of 97.2% and a delaminationof 0.32 N/cm.

What is claimed is:
 1. A diffuse reflector comprising at least onenonwoven sheet of fibers wherein the nonwoven sheet has no opticalbrightener coating, has a basis weight of from about 30 gsm to about 275gsm and has a reflectance and a delamination selected from the groupconsisting of: a reflectance of at least about 97.0% and a delaminationof at least about 0.18 N/cm, a reflectance of at least about 96.0% and adelamination of at least about 0.49 N/cm, and a reflectance of at leastabout 95.0% and a delamination of at least about 0.70 N/cm.
 2. Thediffuse reflector of claim 1, wherein the fibers comprises a polyolefinpolymer.
 3. The diffuse reflector of claim 2, wherein the polyolefinpolymer is polyethylene.
 4. The diffuse reflector of claim 1, whereinthe fibers are flash spun plexifilamentary fibers.
 5. The diffusereflector of claim 1, wherein the at least one nonwoven sheet has anoptical brightener coating.